204 DIFFICULTIES ON THEOKY. Chap. VI. 



the two forms which it connects ; consequently the two 

 latter, during the course of further modification, from 

 existing in greater numbers, will have a great advantage 

 over the less numerous intermediate variety, and will 

 thus generally succeed in supplanting and extermi- 

 nating it. 



We have seen in this chapter how cautious we should 

 be in concluding that the most different habits of life 

 could not graduate into each other; that a bat, for 

 instance, could not have been formed by natural selec- 

 tion from an animal which at first could only glide 

 through the air. 



We have seen that a species may under new condi- 

 tions of life change its habits, or have diversified habits, 

 with some habits very unlike those of its nearest con- 

 geners. Hence we can understand, bearing in mind 

 that each organic being is trying to live wherever it 

 can live, how it has arisen that there are upland geese 

 with webbed feet, ground woodpeckers, diving thrushes, 

 and petrels with the habits of auks. 



Although the belief that an organ so perfect as the 

 eye could have been formed by natural selection, is 

 more than enough to stagger any one ; yet in the case 

 of any organ, if we know of a long series of gradations 

 in complexity, each good for its possessor, then, under 

 changing conditions of life, there is no logical impossi- 

 bility in the acquirement of any conceivable degree of 

 perfection through natural selection. In the cases in 

 which we know of no intermediate or transitional states, 

 we should be very cautious in concluding that none 

 could have existed, for the homologies of many organs 

 and their intermediate states show that wonderful meta- 

 morphoses in function are at least possible. For instance, 

 a swim-bladder has apparently been converted into an 

 air-breathing lung. The same organ having performed 



